A number of mandrel and expanding dowel devices have been previously disclosed. The prior art has several drawbacks which are exemplified in those patents disclosed below.
The stability of the grasp of the mandrel, and the accuracy of the location of the device within the inner diameter of a workpiece is directly related to the number of contact points between the two elements. Most of the prior art mandrels disclose only two point contact. For example, they have two circular or parabolic elements which are slidably engaged in a wedge configuration which come into contact with the inner diameter of the workpiece at the apex of each of the semi-circular mandrel elements. This type of configuration is exemplified by Cunningham, U.S. Pat. No. 1,683,167; Brown, U.S. Pat. No. 3,117,483; and the first embodiment of Liebhard, U.S. Pat. No. 4,464,076.
Another problem encountered in prior art mandrels is the method of actuation. Some, like Cunningham, have external handles or actuation mechanisms which present a problem if space is a consideration. These methods of actuation also may prevent rotational motion about a central axis if the workpiece must be spun while within the grasp of the mandrel. Hohwart, et. al, U.S. Pat. No. 3,833,229, exemplifies a mandrel which cannot be rotated even without an external activation mechanism.
Lastly, the number of moving pieces to obtain the maximum number of contact points is important. A smaller number of moving pieces reduces the error of misaligned mandrel elements, while a greater number of contact points increases the accuracy of centering the mandrel within the workpiece. While Hohwart, et. al., U.S. Pat. No. 3,833,229, utilizes a three-point contact, it is accomplished by four elements, a base and three moving wedges. Liebhard, U.S. Pat. No. 4,464,076, discloses a three point contact having three moving elements in the second embodiment shown.